1. Field of the Invention
The invention relates to a lamp unit for a vehicular headlamp, such as a head lamp, a fog lamp and a position lamp, and, more particularly, to a projector-type lamp unit that uses a light-emitting element, such as a light-emitting diode, as a light source.
2. Description of the Related Art
In recent years, a lamp unit that uses a light-emitting element, such as a light-emitting diode, is increasingly employed as a vehicular headlamp.
For example, FIG. 7 illustrates a lamp unit described in Japanese Patent Application Publication No. 2007-80606. The lamp unit includes a projection lens 2, a light-emitting element 4 and a reflector 6. The projection lens 2 is arranged in an optical axis L that extends in a vehicle longitudinal direction. The light-emitting element 4 is a light source and is arranged to face downward near the optical axis L on the rear side with respect to a rear focal point F of the projection lens 2. The reflector 6 is arranged so as to cover the light-emitting element 4 from the lower side toward which the light-emitting element 4 irradiates light, and reflects the light irradiated from the light-emitting element 4 forward to the optical axis L.
Then, the reflector 6 is formed in an elliptical shape in longitudinal section and has a first focal point f1 at the center of light emission of the light-emitting element 4 and a second focal point f2 at the rear focal point F of the projection lens 2. In order to effectively utilize light reflected by (an effective reflective surface of) the reflector 6, light reflected at a front edge portion (portion including an edge adjacent to the projection lens 2) 6a of (the effective reflective surface of) the reflector 6 is allowed to enter the projection lens 2. That is, the front edge portion 6a of (the effective reflective surface of) the reflector 6 is a limit point for introducing light from the light-emitting element 4 toward the projection lens 2, and is naturally determined on the basis of the size of the projection lens 2 and the position of the rear focal point F.
However, because an axis that passes through the first and second focal points f1 and f2 of the reflector 6 (major axis of the elliptical shape of the reflector 6) is aligned along the optical axis L, when taking into consideration light reflected at the reflector front edge portion 6a, the ratio b/a of a distance b from a reflective position of the reflector 6 to the second focal point f2 with respect to a distance a from the center of light emission to the reflective position is relatively large. Therefore, a light source image projected onto a light distribution screen (not shown) located forward of the projection lens 2 is magnified to thereby relatively widen a light condensing area. As a result, the luminous intensity of a hot zone at the center portion of a distribution pattern formed by the lamp unit is insufficient.
Then, in the lamp unit, an additional reflective surface (downward facing reflective surface) 8 that reflects part of light reflected by the reflector 6 toward the projection lens 2 is provided between the reflector 6 and the projection lens 2. By so doing, a second light distribution Ls formed by the additional reflective surface (downward facing reflective surface) 8 is added to a first light distribution Lm formed by the reflector 6 to thereby increase the luminous intensity of the hot zone (compensate for the insufficient luminous intensity of the hot zone).
That is, in the lamp unit, as shown in FIG. 7 and FIG. 8, the light distribution Lm (first distribution pattern Pm) of light reflected by the reflector 6 is combined with the light distribution Ls (second distribution pattern Ps) of light reflected by the additional reflective surface 8 to thereby obtain a desired high beam distribution pattern of which the luminous intensity of the center hot zone is increased. Note that the portion indicated by the broken line in FIG. 8 shows a light shielding region that is cut by the front edge portion of the additional reflective surface (downward facing reflective surface) 8.
In the lamp unit, light reflected by the additional reflective surface (downward facing reflective surface) 8 provided between the reflector 6 and the projection lens 2 is utilized as the light distribution Ls (part of light reflected by the reflector 6 is controlled by the downward facing reflective surface 8) to thereby make it possible to increase the luminous intensity of the hot zone.
However, in this case, light that forms the second distribution pattern Ps (second light distribution) Ls loses energy when the light is reflected by the reflector 6 and the downward facing reflective surface 8 twice, and has a low intensity. Therefore, light irradiated from the light-emitting element 4 is not effectively utilized because of the loss of energy. That is, the effective utilization of light irradiated from the light-emitting element 4 is low.
Furthermore, because of the additional reflective surface (downward facing reflective surface) 8, the distribution pattern (see FIG. 8) having a cut-off line A is formed at the lower side. Thus, the contrast is apparent along the cut-off line A. This may possibly cause deterioration in forward visibility.